There are three main groups of rock drilling methods: percussive, rotary crushing and cutting rock drilling. In percussive and rotary crushing rock drilling the bit buttons are working as rock crushing tools as opposed to cutting rock drilling, where the inserts work rather as cutting elements. A rock drill bit generally consists of a body of steel which is provided with a number of inserts comprising cemented carbide. Many different types of such rock bits exist having different shapes of the body of steel and of the inserts of cemented carbide as well as different numbers and grades of the inserts.
For percussive and rotary crushing rock drilling, the inserts often have a rounded shape, generally of a cylinder with a rounded top surface, generally referred to as a button.
For cutting rock drilling, the inserts often are provided with an edge acting as a cutter.
There already exists a number of different high pressure/high temperature (HP/HT) sintered cutters provided with polycrystalline diamond layers. These high wear resistant cutter tools are mainly used for oil drilling. The technique when producing such polycrystalline diamond tools using high pressure/high temperature has been described in a number of patents, e.g.:
U.S. Pat. No. 2,941,248: "High Temperature High Pressure Apparatus". U.S. Pat. No. 3,141,746: "Diamond Compact Abrasive". High pressure bonded body having more than 50% by volume diamond and a metal binder: Co, Ni, Ti, Cr, Mn, Ta, etc. These patents disclose the use of a pressure and a temperature where diamond is the stable phase.
In some later patents: e.g., U.S. Pat. Nos. 4,764,434 and 4,766,040, high pressure/high temperature sintered polycrystalline diamond tools are described. In the first patent, the diamond layer is bonded to a support body having a complex, non-plane geometry by means of a thin layer of a refractory material applied by PVD or CVD technique. In the second patent, temperature resistant abrasive polycrystalline diamond bodies are described having different additions of binder metals at different distances from the working surface.
A recent development in this field is the use of one or more continuous layers of polycrystalline diamond on the top surface of the cemented carbide button. U.S. Pat. No. 4,811,801 discloses rock bit buttons including such a polycrystalline diamond surface on top of the cemented carbide buttons having a Young's module of elasticity between 80 and 102.times.106.sup.6 p.s.i., a coefficient of thermal expansion between 2.5 and 3.4.times.10.sup.-6 .degree.C.sup.-1, a hardness between 88.1 and 91.1 HRA and a coercivity between 85 and 160 Oe. Another development is disclosed in U.S. Pat. No. 4,592,433, including a cutting blank for use on a drill bit comprising a substrate of a hard material having a cutting surface with strips of polycrystalline diamond dispersed in grooves, arranged in various patterns.
U.S. Pat. No. 4,784,023 discloses a cutting element comprising a stud and a composite bonded thereto. The composite comprises a substrate formed of cemented carbide and a diamond layer bonded to the substrate. The interface between the diamond layer and the substrate is defined by alternating ridges of diamond and cemented carbide which are mutually interlocked. The top surface of the diamond body is continuous and covering the whole insert. The sides of the diamond body are not in direct contact with any cemented carbide.
Another development in this field is the use of cemented carbide bodies having different structures in different distances from the surface. U.S. Pat. No. 4,743,515 discloses rock bit buttons of cemented carbide containing eta-phase surrounded by a surface zone of cemented carbide free of eta-phase and having a low content of cobalt in the surface and a higher content of cobalt closer to the eta-phase zone. U.S. Pat. No. 4,820,482 discloses rock bit buttons of cemented carbide having a content of binder phase in the surface that is lower and in the center higher than the nominal content. In the center there is a zone having a uniform content of binder phase. The tungsten carbide grain size is uniform throughout the body.